CHAPTER 6 - The Production of the Primary Beam (X-rays)

Summary

Introduction

In this and the next chapter we shall study the methods which are used to ensure that primary beams of the desired wavelength and divergence fall upon the specimen. Since these methods are quite different for X–rays and neutrons we shall here depart from our usual procedure and consider X–rays in this chapter and neutrons in the next.

X–ray intensity measurements are generally made with radiation from an X–ray tube with a copper or molybdenum target. In many investigations it is sufficient to remove the Kβ component from the direct beam by simple filtration: this procedure is discussed in §6.4. In addition to the Kα–doublet the incident beam then still contains a considerable proportion of non–characteristic ‘white’ radiation, whose effect is suppressed by the use of balanced filters, also treated in §6.4.

With either method of filtration the direct beam from the X–ray target strikes the crystal. The beam divergence is determined entirely by the dimensions of the source collimator, the crystal and the focal spot; suitable dimensions of these are discussed in §§6.2 and 6.3.

The highest degree of monochromatization occurs when the primary beam is reflected from a crystal monochromator. In this case the divergence of the beam which falls on the specimen is in part determined by the mosaic spread of the monochromator. Monochromators are discussed in §6.5.

X–ray sources

It is not proposed to discuss the production and properties of X–rays in the present monograph, as these subjects have been fully covered elsewhere.